Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase. 2-Butanone, also referred to as methyl ethyl ketone (MEK), is a widely used solvent and is the most important commercially produced ketone, after acetone. It is used as a solvent for paints, resins, and adhesives, as well as a selective extractant, activator of oxidative reactions, and it can be chemically converted to 2-butanol by reacting with hydrogen in the presence of a catalyst (Nystrom, R. F. and Brown, W. G. (J. Am. Chem. Soc. (1947) 69:1198). 2,3-butanediol may be used in the chemical synthesis of butene and butadiene, important industrial chemicals currently obtained from cracked petroleum, and esters of 2,3-butanediol may be used as plasticizers (Voloch et al. Fermentation Derived 2,3-Butanediol, in Comprehensive Biotechnology, Pergamon Press Ltd, England Vol 2, Section 3:933-947 (1986)).
Microorganisms may be engineered for expression of biosynthetic pathways for production of 2,3-butanediol, 2-butanone, 2-butanol and isobutanol. Commonly owned and co-pending U.S. Patent Application publication U.S. 20070092957 A1 discloses the engineering of recombinant microorganisms for production of isobutanol. Commonly owned and co-pending U.S. Patent Application publications U.S. 20070259410A1 and U.S. 20070292927A1 disclose the engineering of recombinant microorganisms for production of 2-butanone or 2-butanol. Multiple pathways are disclosed for biosynthesis of isobutanol and 2-butanol, all of which initiate with cellular pyruvate. Butanediol is an intermediate in the 2-butanol pathway disclosed in commonly owned and co-pending U.S. Patent Application publication U.S. 20070292927A1.
Production of 2,3-butanediol, 2-butanone, 2-butanol and isobutanol in recombinant yeasts is limited by availability of substrate flow from natural yeast metabolic pathways into engineered biosynthetic pathways producing these compounds. Since the biosynthetic pathways for isobutanol, 2,3-butanediol, 2-butanol, and 2-butanone draw from host cell production of pyruvate, this substrate may be a limitation in product formation. The first step in these engineered pathways is conversion of pyruvate to acetolactate, which is catalyzed by acetolactate synthase.
Pyruvate metabolism has been altered in yeast for production of lactic acid and glycerol. U.S.20070031950 discloses a yeast strain with a disruption of one or more pyruvate decarboxylase or pyruvate dehydrogenase genes and expression of a D-lactate dehydrogenase gene, which is used for production of D-lactic acid. Ishida et al. (Biosci. Biotech. and Biochem. 70:1148-1153 (2006)) describe Saccharomyces cerevisiae with disrupted pyruvate dehydrogenase genes and expression of lactate dehydrogenase. U.S.2005/0059136 discloses glucose tolerant C2 carbon source-independent (GCSI) yeast strains with no pyruvate decarboxylase activity, which may have an exogenous lactate dehydrogenase gene. Nevoigt and Stahl (Yeast 12:1331-1337 (1996) describe the impact of reduced pyruvate decarboxylase and increased NAD-dependent glycerol-3-phosphate dehydrogenase in Saccharomyces cerevisiae on glycerol yield.
To improve the production of isobutanol, 2,3-butanediol, 2-butanol or 2-butanone in yeast, the problem remaining to be solved is to increase the conversion of pyruvate to acetolactate, which then flows into engineered biosynthetic pathways to produce these compounds.